Integrated circuits (ICs) can be digital, analog or mixed. Typically, the design and manufacture of digital ICs is different from the design and manufacture of analog ICs. Therefore, mixed ICs generally require additional production effort and costs.
An example of an analog device that can be provided on an IC is the operational amplifier (“op-amp”). An op-amp is a DC-coupled, high gain electronic voltage amplifier with differential inputs including a V+ input and a V− input. The output voltage of the op-amp is given by the equation Vout=AOL(V+−V−), where V+ is the voltage at the non-inverting terminal, V− is the voltage at the inverting terminal and AOL is the open-loop gain of the amplifier.
Since the magnitude of AOL is typically very large (e.g. 100,000 or more) and therefore even a quite small difference between V+ and V− (“the differential voltage”) drives the amplifier output nearly to the supply voltage. Situations in which the output voltage is equal or greater than the supply voltage are referred to as saturation of the amplifier and can be dangerous to both the op-amp and the system to which it forms a part. If predictable operation is desired, negative feedback is used, by applying a portion of the output voltage to the inverting input. The closed loop feedback greatly reduces the gain of the amplifier.
Digital integrated circuits can be tested with a technique known as built-in self-test (BIST). BIST techniques have been used for on-line testing (during the normal operation of the IC) and off-line testing (during a test mode operation of the IC). Structures used for on-line and off-line BIST are different. This is because on-line BIST uses normally occurring data as inputs and employs redundancy techniques such as information redundancy, time redundancy, and hardware redundancy to do on-line checking. Off-line BIST, on the other hand, uses stored and/or generated test vectors as inputs and employs compression and comparison techniques.
Analog integrated circuits, such as those incorporating op-amp devices, can be critical components in many electronic systems. Furthermore, some analog ICs, particularly those used in industrial applications, can be relatively high power devices. In those situations where analog devices are not operating within proper parameters it is possible for those devices to become damaged and/or cause damage to other components. In general, analog ICs do not have comparable techniques to either the off-line or the on-line BIST testing of digital ICs.
These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.